Design of Micropiles for Slope Stabilization

Transcription

1 Design of Micropiles for Slope Stabilization J. Erik Loehr, Ph.D., P.E. University it of Missourii ADSC Micropile Design and Construction Seminar Las Vegas, Nevada April 3-4, 8 Outline Background Typical implementation Construction sequence Stability Analysis Issues Prediction of resistance for micropiles Comparison of predicted and measured resistance Summary and conclusions 2 1

2 In-situ reinforcement schemes Soil Dowels Fill Shotcrete Soil Nil Nails Relic Shear Surface Stiff Clay Firm Stratum Reticulated Micropiles 3 after Bruce and Jewell, 1986 Common implementation micropiles anchor 4 2

3 Oso Creek Landslide Stabilization 5 Photo courtesy of John Wolosick Stability analysis for reinforced slopes Potential Sliding Surface R R axial R lat Reinforcing Member 6 3

4 Stability analysis for reinforced slopes Same methods of analysis used for reinforced slopes Same assumptions invoked Same solution methods used Only change is to include magnitude of known force(s) into equilibrium calculations Force must be consistent with breadth considered in stability analyses (i.e. force/unit width) Search for critical sliding surface can also become more challenging Magnitude of resisting force changes with location Numerous local minima frequently exist 7 Example Micropiles battered at +/- 45 deg. F=1. Fill weathered shale 8 4

5 Result F= K 34 K 11 K 37 K Fill weathered shale 9 Stresses on sliding surface Effective Normal Stress (psf) Mobilized Shear Resistance (psf) increase in stress due to upslope p pile Stress (psf) decrease in stress due to downslope pile X coordinate (ft) 5

6 Impact of reinforcement on stability Reinforcement contributes to stability in two ways: Direct resistance to sliding Modifying normal stress on sliding surface Both of these can be significant Relative magnitude of contributions depends on: Orientation of reinforcement w.r.t. soil movement Type of reinforcement Depth of sliding Frictional resistance of soil 11 Prediction of micropile resistance Potential sliding surface 12 6

7 The challenge Micropiles are passive elements Soil provides both load and resistance load transfer is complex Numerous limit states Must consider compatibility of axial and lateral resistance Must be able to mobilize resistance within tolerable deformations 13 Prediction of micropile resistance Estimate profile of soil movement Resolve soil movement into axial and lateral components Independently predict mobilization of axial and lateral resistance Using p-y analyses for lateral load transfer Using t-z analyses for axial load transfer Select appropriate axial and lateral resistance with consideration given to compatibility and serviceability 14 7

8 Soil movement components + θ θ Slope Surface α θ δ axial δ soil δlat. δ lat. axial δ soil α θ δ 15 Sliding Surface p-y analyses for lateral resistance L-Pile Model Input Profile of Lateral Soil Movement δ lat Lateral Component of moving soil Soil Lateral Resistance (p) Pile Bending Stiffness (EI) Sliding Surface Transition (Sliding) Zone Stable Soil (no soil movement) z 16 8

9 Lateral resistance from p-y analyses Use soil movement option (L-Pile v4.m or v5) For an assumed depth of sliding: 1. Apply displacements in soil above sliding surface 2. Determine response from p-y analyses 3. Mobilized resistance is shear force in micropile at depth of sliding 4. Repeat steps 1 through 3 with incrementally increasing displacement until a limit state is reached Shear force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth NOTE: MUST ALSO CONSIDER DEFORMATIONS REQUIRED TO MOBILIZE RESISTANCE 17 Mobilization of lateral resistance Pile Deformation (in) Mobilized Bending Moment (kip-in) Mobilized Shear Force (kip) clay 1 1 d=.1 in d=1. in d=3. in Depth (ft) 3 slide rock

10 Mobilization of lateral resistance Mobilized Shear Force (kip) Total Slope Movement (in) t-z analyses for axial resistance Input Profile of Axial Soil Movement Cap Bearing δ axial Axial Component of moving soil Soil Shear Resistance (t) Pile Axial Stiffness (EA) Sliding Surface Transition (Sliding) Zone Stable Soil (no soil movement) Soil End Bearing (Q) z 1

11 Axial resistance from t-z analyses For an assumed depth of sliding: 1. Apply displacements in soil above sliding surface 2. Determine response from t-z analyses 3. Mobilized resistance is axial force in shaft at depth of sliding 4. Repeat steps 1 through 3 with incrementally increasing displacement until a limit state is reached Axial force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth NOTE: MUST ALSO CONSIDER DEFORMATIONS REQUIRED TO MOBILIZE RESISTANCE 21 Mobilization of axial resistance Depth (ft) Mobilized Axial Load (kip) d=.1 in d=.3 in d=.42 in d=.5 in clay slide rock

12 Mobilization of axial resistance Mobilized Axial Force (kip) Total Slope Movement (in) Limit states for soil reinforcement Soil failure passive failure (lateral) above or below sliding surface pullout failure (axial) above or below sliding surface Structural failure flexural failure shear failure axial failure - compression - tension Serviceability limits 24 12

13 Repeat for other sliding depths Result is two resistance functions that describe resistance versus position along reinforcement 25 Resistance functions (per member) Axial Resisting Force (kip) Lateral Resisting Force (kip) clay 1 Ultimate d<1-in Sliding Depth (ft) 3 Sliding Depth (ft) rock Member resistance for individual member 13

14 Input for stability analyses (per lineal foot) Axial Resisting Force (kip/ft) Lateral Resisting Force (kip/ft) spacing = 6-ft 1 clay 1 Sliding Depth (ft) 3 Sliding Depth (ft) rock Member resistance divided by member spacing Comparison w/ measured values 28 14

15 Mobilized bending moments Littleville Bending Moment (in-kips) -4-4 Bending Moment (in-kips) predicted measured (2+7U) measured (1+7U) upslope p mod =.2 1 predicted measured (2+7U) measured (1+7U) downslope p mod =.2 Depth (ft) 3 Depth (ft) δ tot =.39-in 5 δ tot =.31-in 29 Mobilized axial resistance Littleville Axial Load T, kip (+=tension) Axial Load T, kip (+=tension) Depth, z (ft.) upslope α =.3 z ult =.6-in predicted measured (2+7U) measured (1+7U) δ tot =.34-in Depth, z (ft.) δ tot =.24-in downslope α =.3 z ult =.6-in predicted measured (2+7U) measured (1+7U) 3 15

16 Large-scale model tests 31 Model vs. measurement no cap 3 3 ion Along Pile (in. from bottom) (2.8) LPile (2.8) on Along Pile (in. from bottom) (2.8) t-z (2.8) Positi 5 Positi Induced Bending Moment (lb-in) C Induced Axial Load (lb) T 32 Test 2-A, Member 3 (downslope), S/D=1 16

17 Model vs. measurement with cap ion Along Pile (in. from bottom) Positi (1.9) LPile (1.9) Positio on Along Pile (in. from bo ottom) (1.9) t-z (1.9) Induced Bending Moment (lb-in) C Induced Axial Load (lb) T 33 Test 3-A, Member 2 (upslope), S/D=1 Summary and Conclusions Prediction of resistance for reinforcement requires consideration of soil-structure interaction Cannot predict resistance based on structural capacity alone!!! Both axial and lateral components of resistance can substantially influence stability Relative contribution depends on pile orientation and pile/soil characteristics Axial resistance frequently mobilized at relatively small soil movements Lateral resistance frequently requires greater soil movements Uncoupled method suitable for predicting resistance when no cap or when cap influence is limited Comparison of measured and predicted forces reasonable BUT may need to use modified p-y and t-z models Additional data needed!!! 34 17

18 Acknowledgements ADSC/DFI Micropile Committee ADSC Industry Advancement Fund National Science Foundation Grant CMS92164 Many students 35 18